1. Introduction
Reducing energy consumption and minimizing environmental pollution are crucial concerns globally, with the construction industry being a major contributor to energy consumption and carbon emissions [1]. According to statistics, heating accounts for over 75% of the final energy consumption in the building and industrial sectors [2]. Therefore, reducing the energy consumption of buildings and their associated greenhouse gas (GHG) emissions is essential for combating global warming. Currently, residents in cold regions generally use traditional heating methods in winter, such as burning coal and wood. However, these methods consume a lot of energy, cause significant pollution, and negatively impact residents’ health [3]. Some buildings utilize ground-source heat pumps, solar thermal systems, air-source heat pumps, and passive design strategies for heating [4]. However, ground-source heat pumps have high costs and site limitations [5], solar thermal systems are prone to pipe freezing and rupture in low-temperature environments [6], and air-source heat pumps can frost over in cold conditions, increasing operational costs [7]. Passive design strategies are typically used to reduce heating demands but rarely meet the full heating load of buildings [8]. However, as one of the most widely used low-carbon heating technologies, air-source heat pumps (ASHPs) consume 55–70% less energy than electric heating systems and emit 12% less CO2 than gas boilers [9]. Considering the equipment costs, system complexity, and operational stability, combining passive design strategies with air-source heat pump systems holds great potential for winter heating in cold regions [10].
Passive design strategies typically involve improving building conditions based on the local climate to optimize indoor environments while reducing energy demand [11]. Many researchers have proposed various passive strategies for cold conditions, such as optimizing building shape and orientation [12], enhancing building airtightness [13], passive solar heating [14], optimizing door and window materials [15], using phase-change materials (PCMs) [16,17], and Trombe walls [18,19]. Solar energy is widely distributed in nature; sunspaces, as a passive solar heating technology, have significant advantages in terms of energy saving, environmental protection, and sustainable development [20]. During building operation, sunspaces can collect solar energy to increase indoor temperatures and heat adjacent rooms [21]. Yang et al. [22] studied the improvement of rural cave dwellings with sunspaces, showing that adding sunspaces can increase the air temperature of the main room by 1.0 °C on weekdays and by 4.3 °C on holidays. Li et al. [23] evaluated the heating performance of sunspaces in rural houses in Lhasa (a cold region with abundant solar resources) through measurements. The results showed that the average temperature of the bedrooms in the south-facing rooms reached 16.91 °C, and the sunspaces in these rooms alone could meet the heating needs. Ma L et al. [24] used Energy Plus to simulate the heating performance of sunspaces in rural houses in Anda City (a cold region with scarce solar resources). They found that compared to reference buildings, the heating energy consumption of sunspaces was reduced by 4.6%. However, sunspaces as a passive heating strategy struggle to heat all rooms and have fluctuating solar energy availability throughout the day. Therefore, in the face of high heating demands in cold regions, active technologies such as air-source heat pumps are also needed in addition to passive strategies.
Air-source heat pumps (ASHPs) are typically compression-type heat pumps that rely on the Carnot cycle theorem, using outdoor air as the heat source. By consuming a small amount of high-grade energy such as electricity, they produce a large amount of low-grade heat energy, achieving energy savings [25]. In cold regions, the heating performance of ASHPs during winter is mainly limited by ambient temperature. At low temperatures, the coefficient of performance (COP) and heating capacity of the heat pump are affected [26]. Currently, there are many methods to improve the low-temperature adaptability of ASHPs, such as using scroll compressors with flash tanks [27], adopting two-stage compression systems [28], using cascade ASHP compression [29], and optimizing outdoor coil fins [30]. However, these methods still cannot effectively solve the problems faced by ASHPs under extremely low temperatures. Many researchers have proposed the use of solar-assisted air-source heat pump (SAASHP) systems for combined heating, which collect and utilize solar energy to compensate for and reduce the impact of ambient temperature on ASHP heating performance [31]. Abbasi B et al. [32] conducted a study that mainly explored a thermal performance comparison between glass-covered and non-glass-covered direct expansion solar-assisted heat pump (DX-SAHP) water heaters, and examined the system performance under different ambient temperatures and solar radiation intensities. The results showed that the non-glass-covered system had a higher COP throughout the year, especially during winter when sunlight is scarce. The glass-covered system could only protect the collector plate from heat loss in winter, and the payback period of the system was less than five years. Li et al. [33] conducted a comparative analysis using TRNSYS 16 software to simulate the performance of a system combining an air-source heat pump (ASHP) and a solar evacuated-tube water heater (SETWH) with that of a system combining a micro heat pipe photovoltaic/thermal system (MHP-PV/T). The MHP-PV/T-ASHP system increased the solar fraction and system efficiency ratio by 19% and 2.2 times, respectively, and improved the primary energy saving rate by 12.3%. Han et al. [34] studied the performance of solar and heat pump combined heating systems. The results showed significant energy-saving effects compared to traditional distributed coal-fired heating and hot water supply, with a maximum of 43.55%. Although the heating performance of solar-coupled heat pump systems has been widely discussed, current combined heating systems mainly use water for heat storage. However, due to the instability and discontinuity of solar energy supply throughout the day, there is still room for improvement in 24 h heating.
Some scholars have found that integrating phase change heat storage (PCHS) with solar-assisted air-source heat pump (SAASHP) systems can improve the heating stability and efficiency of SAASHP systems by leveraging PCHS technology [35]. These methods include incorporating phase change materials (PCMs) with the evaporation side, condensation side, or heat exchanger of the SAASHP system [36]. Han et al. [37] investigated the performance of a solar and heat pump combined heating system based on PCHS and found that it could save up to 43.55% of energy compared to traditional distributed coal-fired heating and hot water supply systems. J. Gao et al. [38] proposed a solar-coupled air-source heat pump system with phase change heat storage. Their experiments demonstrated that the average power consumption of the compressor was 1.87 kW, a reduction of 21.1%, and the system’s average coefficient of performance was 5.42, an increase of 143.0% over the original system. Qu et al. [39] discovered that incorporating an annular energy storage heat exchanger and solar energy into the heat pump system significantly improved the thermal performance under low-temperature conditions. Additionally, Ni et al. [40] conducted experimental research on the annual operation of a solar and ASHP heating system using a three-pipe energy storage heat exchanger. They found that higher temperatures and flow rates of hot water on the solar side could significantly enhance the heating energy efficiency of the system.
The existing literature shows that extensive research has been conducted on passive heating and the optimization of ASHP-based systems. However, these systems are complex and have issues with operational stability. To address these problems, we propose a coupling heating scheme that integrates an ASHP with passive heating. This system mainly consists of an air-source heat pump unit, a heat storage tank, heating terminals, and a passive solar room. The air-source heat pump provides heating for the building, while the passive solar room creates suitable working conditions for the heat pump. During the daytime in winter, the passive solar room collects solar energy, raising the internal temperature and providing a favorable working environment for the air-source heat pump [41]. The high-temperature air inside the passive solar room serves as the heat source for the heat pump, requiring less air to obtain the same amount of heat compared to outdoor operation, thereby significantly improving the heating efficiency of the air-source heat pump. Additionally, the favorable ambient temperatures reduce the likelihood of frost formation on the heat pump’s evaporator. To address the issue of insufficient solar energy at night, we propose using phase change materials and heat storage media in the building to store excess heat during the day and release it at night, thereby meeting the heating demand throughout the day. This study focuses on three key aspects: analyzing the heating performance of coupled systems on the coldest day of winter in severe cold regions and evaluating their energy supply capacity and power consumption; comparing the energy consumption and economic benefits of coupled systems with traditional ASHP systems to verify their energy-saving effects; and exploring the application of phase change materials in heating systems to enhance system stability and sustainability through heat storage and release mechanisms. The working principle of the system is illustrated in Figure 1. This paper uses software such as DesignBuilder and TRNSYS for simulation and mathematical analysis, comparing the coupling scheme with traditional schemes to explore the heating performance and energy-saving effects of the ASHP and passive heating coupling system in rural houses in extremely cold regions.
2. Methods
The research object is a single-story rural house in Harbin, China (latitude: 46.5°, longitude: 128.44°). The floor plan of the house follows a typical square layout [42], as shown in Figure 2. Located in the severely cold region of China’s architectural thermal zones, the house measures 15.7 m in length (east to west) and 10.2 m in width (north to south), with a ceiling height of 3.3 m. The main bedroom and living space are combined and situated on the southwest side of the building. The east side serves as a storage room, and the kitchen and secondary bedroom are located in the middle. An additional solar room is set on the south side of the building, measuring 15.7 m by 3 m and 2.7 m in height. This solar room features hollow transparent glass on its east, west, and south sides as well as on the roof, supported by an aluminum alloy frame. The south exterior wall of the living room has an exterior window measuring 3.3 m by 1.5 m, with a sill height of 0.9 m. The north exterior wall has an exterior window measuring 3.0 m by 1.5 m. The kitchen’s south side has an exterior door measuring 0.9 m by 2.0 m, and the north side of the secondary bedroom has an exterior window measuring 2.4 m by 1.5 m. There are also interior doors measuring 0.8 m by 2.0 m between the living room and kitchen and between the kitchen and secondary bedroom. The floor plan of the rural house is shown in Figure 3.
2.1. Building Model Establishment
The basic information of the farmhouse is shown in Table 1.
The designed indoor heating temperature for the rooms is 20.0 °C [43]. The lighting power density is 5 W/m2, the internal personnel density is 0.11 people/m2, and the airtightness infiltration setting is 0.7 ac/h. The rooms are also equipped with other electrical devices, with an average electrical load of 0.650 kW. The U-values of the building envelope are shown in Table 2, complying with the relevant standards. [44].
Based on the above building information, a typical rural house model was constructed using DesignBuilder 7.0.2 software, with parameter settings for the thermal performance of the building envelope. The model is shown in Figure 4.
2.2. Air-Source Heat Pump Model Establishment
Based on the typical meteorological conditions of Harbin, the simulation period is set for January, the coldest month in Harbin (1 January to 31 January). The typical annual hourly dry-bulb temperature data were extracted from the EnergyPlus website (
A simulation analysis of the building’s heat load during the heating period was conducted using DesignBuilder software to select and calculate the air-source heat pump. The hourly heat load of the building is illustrated in Figure 6. The cumulative heat load during the operation period is 9271.412 kWh, with the maximum hourly heat load reaching 5.99 kWh.
Based on the above building parameters and heat load data analysis, the air-source heat pump model RY14-NcPB5U1 was selected for this study. The designed supply and return water temperatures are 45/40 °C, with a minimum operating temperature of −25 °C. Radiant heating is used as the terminal heating method. The specific parameters of the heat pump system are shown in Table 3. The parameters of the air-source heat pump were set and modeled using DesignBuilder software, as shown in Figure 7 and Figure 8.
2.3. Simulation and Analysis
This study analyzes two operating conditions: Mode 1 and Mode 2. Both modes use the aforementioned Harbin typical rural house building parameters and the RY14-NcPB5U1 model ASHP parameters for simulation analysis. Mode 1 is the traditional air-source heat pump with the outdoor unit placed outside the building for heating. Mode 2 is passive heating combined with the air-source heat pump system. The operating effects and energy consumption of the two modes are compared and analyzed to evaluate the feasibility of the passive heating combined with the air-source heat pump scheme.
2.3.1. Simulation and Analysis of Traditional Mode (Mode 1)
Using the simulation system built with DesignBuilder, the air-source heat pump’s outdoor unit is placed outside the building. The operating temperature is the outdoor dry-bulb temperature, and the air-source heat pump is controlled to start and stop based on the indoor temperature, running when the indoor temperature is below 20 °C and stopping when it is above 20 °C. Additionally, when the outdoor temperature is too low and the air-source heat pump cannot provide enough heat, an electric heater is used to assist in heating the water tank, operating in a circulating control mode with a heat efficiency of 90%.
Based on the above data, the coldest day of winter, January 1st, is selected for analysis. If the heating demand can be met on the coldest day, it can be met on other days as well. The hourly temperature comparison between the heated room and the outdoor temperature on this day is shown in Figure 9.
2.3.2. ASHP and Passive Heating Coupled System Simulation and Analysis (Mode 2)
Mode 2 is the ASHP and passive heating coupled system. For the system model in Mode 2, traditional simulation software cannot perform the operation of placing the heat pump’s outdoor unit in the sunroom. Therefore, this scheme adopts a combination of mathematical models and external files. The heat output of the heat pump is calculated based on a mathematical model supplemented by external files provided by the user.
The energy efficiency of air-source heat pumps is primarily determined by the coefficient of performance (COP), which is the ratio of the heating output to the input power of the heat pump system. Based on the COP and heating output of the ASHP, the input electrical energy of the air-source heat pump can be calculated using the following formula:
(1)
where COP is the coefficient of performance of the air-source heat pump; is the heating output of the air-source heat pump in kWh; and is the input electrical energy of the air-source heat pump in kWh.Meanwhile, the total heating capacity of the heat pump is composed of the electrical energy consumed by the heat pump and the heat absorbed from the environment . Therefore, by knowing the electrical energy consumption and the total heating capacity, we can calculate the heat input from the surrounding environment into the heat pump. The calculation formula for is as follows:
(2)
where is the heat input of the air-source heat pump in kWh.The same ASHP model RY14-NcPB5U1 as in Mode 1 is used. The ASHP model is built using Trnsys 16 software, with the supply and return water temperature set to 45/40 °C. The relationship curve between the COP (coefficient of performance) and ambient temperature for this ASHP model is shown in Figure 10.
The simulation results of internal temperatures in the passive heating sunroom (with no heating terminal inside) on the coldest day, January 1st, using DesignBuilder software are shown in Figure 11. The hourly solar energy acquisition curve for the sunroom is depicted in Figure 12.
According to the simulation results, the sunroom receives solar energy from 8:00 to 17:00. By 9:00, the sunroom temperature reaches 0.22 °C Assuming the air-source heat pump starts operating from 9:00, all the energy obtained by the sunroom is converted into heat to provide indoor heating, maintaining the sunroom temperature at 0.22 °C. Based on the relationship curve between the COP of the air-source heat pump and the ambient temperature, at this temperature, the COP of the heat pump is approximately 2.69, with a maximum heat output of 14 kW for this model of air-source heat pump. It is calculated that the heat pump’s maximum hourly heat absorption is 8.8 kWh. The hourly heat gains in the sunroom are presented in Table 4, and the sunroom accumulates a total of 152.76 kWh of solar energy throughout the day.
3. Results and Discussions
3.1. Analysis and Discussion of Operational Results for Mode 1 and Mode 2
The simulation results of Mode 1 are summarized in Table 5. During the specified period, the indoor temperature consistently exceeds 16 °C, with an average temperature of 16.74 °C which is 3.26 °C lower than the design temperature of 20 °C. Under operational conditions, the electric heating consumption of the water tank is 109.24 kWh, far exceeding the air-source heat pump consumption of 2.35 kWh. The heat output of the air-source heat pump is insufficient to meet the building’s heat load, with most of the heating provided by the electric heating water tank, consuming a total of 111.59 kWh and producing 64.88 kWh of heating.
The calculation results for Mode 2 are summarized in Table 6. The air-source heat pump coupled with the additional sunroom system absorbs a minimum of approximately 62.46 kWh of heat per day, which can provide around 99.41 kWh of heat to the interior. Assuming a 10% heat loss during the entire process, the interior can receive approximately 89.48 kWh of heat, exceeding the 86.67 kWh required to maintain an indoor temperature of 20 °C. The system’s power consumption is 36.96 kWh.
The analysis is as follows:
In Mode 1, when the outdoor temperature is above −25 °C, meeting the minimum operating conditions for the air-source heat pump, heating can be performed. However, at lower temperatures, the heat pump’s heating efficiency is reduced. The heating system is jointly provided by the air-source heat pump and the electric heating water tank. When the outdoor temperature is below −25 °C the heat pump cannot operate, stopping its function, and only the electric heating water tank works.
In Mode 2, the additional sunroom continuously captures solar energy during the day, maintaining the sunroom temperature and continuously providing energy for the ASHP’s heating. All the heat required for the building’s heat load is provided by the air-source heat pump, achieving the design target indoor temperature of 20 °C. The input heat of the air-source heat pump is less than the heat gained by the heating sunroom, indicating that under the given conditions, the input heat of the air-source heat pump can be fully supplied by the sunroom, as shown in Figure 13.
Mode 2 has a significant advantage over Mode 1 in reducing energy consumption. Compared to Mode 1, the average power consumption is reduced by 74.63 kWh, and energy consumption is decreased by 66.88%. Moreover, Mode 2 is simpler to operate, with the heat pump outdoor unit placed in the sunroom. Therefore, in practical applications, the ASHP coupled with the passive heating system has great prospects for use in severely cold regions.
3.2. All-Weather Operational Design Strategy and Application
To achieve the all-weather use of air-source heat pump units in severely cold regions, the focus is on how to ensure indoor thermal comfort at night without solar energy. Therefore, excess heat transported by the air-source heat pump during the day should be stored, using a medium to provide heat for heating at night. Based on this idea, two feasible solutions are proposed.
Method 1: The air-source heat pump transfers heat to water, using water as the medium to convey heat to the terminals of each room for heating. When the indoor temperature reaches the target (20 °C), the excess heat is stored in the water tank. After the air-source heat pump stops working at night, the water tank continues to provide heat. This method relies on the heat storage capacity of water (or other mediums with a high specific heat capacity). To meet the heating needs for the entire night, the water tank’s volume requirements are significant and should be chosen based on the heating area during design.
Method 2: Building on Method 1, phase change materials (PCMs) are applied to the building. When the indoor temperature is lower than the PCM temperature, the PCM releases the absorbed heat, delaying the drop in indoor temperature and reducing heating power and energy consumption [45]. When the indoor temperature is higher than the PCM temperature, the PCM reabsorbs heat, helping to maintain the temperature. The main applications of phase change heat storage technology in buildings are as follows:
Placing PCM alongside heating equipment: This method allows the PCM to absorb or release heat, achieving energy savings in heating. When the heating system is operating, the PCM absorbs heat; when the heating system stops, the PCM releases the absorbed heat, delaying the drop in indoor temperature [46], as shown in Figure 14.
-
2.. Applying PCM to the enclosing structure of the heating space: The PCM is influenced by the indoor temperature; it absorbs heat when the indoor temperature is higher than the PCM temperature and releases the absorbed heat when the indoor temperature is lower than the PCM temperature [47], as shown in Figure 15.
-
3.. Adding phase change heat storage materials to the floor can also enhance heating efficiency. When the heating medium flows back to the water tank from the heating terminal, it passes through the phase change heat storage materials inside the floor. At night, when the system stops heating, the phase change materials release the absorbed heat [48], as shown in Figure 16.
Compared to Method 1, adding phase change materials can reduce the heating power during the operation of the air-source heat pump, saving heating energy. Additionally, it can delay the rate of indoor temperature decline, improving the building’s thermal insulation performance.
By using appropriate methods to temporarily store excess heat and release it when needed, it is possible to rely solely on the combination of air-source heat pumps and passive heating during the heating season in severely cold regions. In the design, the relationship between the heating area, heat pump power, and water tank size should be considered to maximize cost savings and reduce heating losses.
4. Conclusions
This paper studies the feasibility of air-source heat pump and passive heating coupled systems for heating in severely cold regions, and analyzes the performance of two modes (Mode 1: traditional air-source heat pump system; Mode 2: passive heating combined with air-source heat pump system) through simulation.
(1) Under the conditions of the coldest day in severely cold regions, the air-source heat pump and passive heating coupled system’s total heat output for the entire day is 99.41 kWh, which can meet the energy requirement of 86.67 kWh to maintain an indoor temperature of 20 °C throughout the day.
(2) Due to the excessively low ambient temperature, the traditional ASHP system cannot meet the normal operating temperature for an extended period, resulting in the need for inefficient electric heating water tanks, consuming large amounts of electricity. In Mode 2, the air-source heat pump’s outdoor unit is placed in a passive sunroom, utilizing the higher ambient temperature inside the sunroom, significantly improving the ASHP’s COP value and thereby reducing the heat pump’s power consumption. Analysis shows that the average power consumption of the air-source heat pump and passive heating coupled system is reduced by 66.88% compared to the traditional system, demonstrating excellent energy-saving effects.
(3) The study shows that to address the issue of not being able to capture solar energy at night, excess heat from passive heating during the day is proposed to be stored for night-time heating, achieving stable heating and energy savings. Compared to traditional heating methods, this system greatly reduces energy consumption while ensuring comfort, further quantifying the application potential of air-source heat pumps in cold regions.
Due to the inability to simulate the operating state of the air-source heat pump in an additional sunroom in this study, continuous dynamic simulation could not be achieved. The reliance on traditional mathematical calculations blurred the fluctuations and losses in heat transfer. Future research should focus on the specific design and simulation of heat storage and release, optimizing the design through real system operation on test benches.
Conceptualization, F.J. and G.L.; Methodology, F.J.; Software, F.J. and C.Z.; Validation, F.J. and C.Z.; Data curation, J.L.; Writing—original draft, F.J.; Writing—review & editing, F.J. and G.L. All authors have read and agreed to the published version of the manuscript.
Data are contained within the article.
The author declares no conflicts of interest.
Footnotes
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Enlarge this image.Figure 2. Harbin Region typical farmhouse: (a) southwest view; (b) south elevation.
Enlarge this image.Figure 9. Hourly temperature comparison between the heated room and outdoor temperature.
Enlarge this image.Figure 10. Relationship curve between COP of air-source heat pump and ambient temperature.
Enlarge this image.Figure 11. Simulation results of passive solar sunroom internal temperature on January 1.
Enlarge this image.Figure 14. Co-location of PCM and heating equipment: (a) application during winter day; (b) application during winter night.
Enlarge this image.Figure 15. Application of PCM in building enclosures of heating spaces: (a) application during winter day; (b) application during winter night.
Enlarge this image.Figure 16. Incorporation of PCM in the floor: (a) application during winter day; (b) application during winter night.
Basic information of a typical farmhouse in Harbin.
| Length (m) | Width (m) | Height (m) | South Window (m2) | North Window (m2) | Exterior Door (m2) | Heating Area (m2) |
|---|---|---|---|---|---|---|
| 15.7 | 7.2 | 3.3 | 12.15 | 15.3 | 4.4 | 57.5 |
Heat transfer coefficients of the enclosure structure in typical farmhouses.
| Name | Structure | U-Value (W/m2·K) |
|---|---|---|
| Exterior Wall | Clay brick wall with external insulation board | 0.40 |
| Interior Wall | Clay brick wall | 1.189 |
| Exterior Door | Metal insulated door | 1.84 |
| Building + Sunroom Windows | 13 mm thick double-glazed window | 1.96 |
| Roof | Insulated sloped roof with steel sheet | 0.25 |
Model RY14-NcPB5U1 air-source heat pump parameters.
| Equipment Name | Parameter | Quantity |
|---|---|---|
| Air-Source Heat Pump | Heating capacity: 14 kW, rated power: 4.2 kW (ambient temperature: 7 °C DB/6 °C WB), operating temperature range: −25 to 43 °C, water flow rate: 1.75 m3/h, water pressure drop: 50 kPa. | 1 |
| Thermal Storage Tank | Capacity 80 L, height 940 mm, bottom diameter 462 mm | 1 |
Hourly heat gain of the sunroom.
| Time | Heat Gain (kWh) | Time | Heat Gain (kWh) |
|---|---|---|---|
| 8:00–9:00 | 2.03 | 13:00–14:00 | 26.84 |
| 9:00–10:00 | 10.77 | 14:00–15:00 | 23.08 |
| 10:00–11:00 | 21.00 | 15:00–16:00 | 14.17 |
| 11:00–12:00 | 26.07 | 16:00–17:00 | 0.86 |
| 12:00–13:00 | 27.94 | 17:00–18:00 | 0.00 |
Operational and energy consumption statistics for Mode 1.
| Building Heating Load (kWh) | Average Indoor Temperature (°C) | Heat Pump Electricity Consumption (kWh) | System Heat Output (kWh) | Water Tank Electricity Consumption (kWh) | Total System Electricity Consumption (kWh) |
|---|---|---|---|---|---|
| 86.67 | 16.74 | 2.35 | 64.88 | 109.24 | 111.59 |
Operational and energy consumption Statistics for Mode 2.
| Building Heating Load (kWh) | Average Indoor Temperature (°C) | Heat Pump Electricity Consumption (kWh) | System Heat Output (kWh) | Water Tank Electricity Consumption (kWh) | Total System Electricity Consumption (kWh) |
|---|---|---|---|---|---|
| 86.67 | 20.00 | 36.96 | 99.41 | 0.00 | 36.96 |
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